World Health Organization (WHO) declared tuberculosis (TB) a global health emergency; a distinction never accorded another disease. Two billion people are infected with Mycobacterium tuberculosis (Mtb) worldwide, leading to over 9 million new TB cases each year and nearly 2 million deaths. A considerable obstacle to TB control is the emergence of drug-resistant disease. Multidrug-resistant strains of Mtb (resistant to at least isoniazid [INH] and rifampicin [RIF]) result in 440,000 new cases of MDR-TB each year in 41 countries. Determination of drug resistance is not routinely made prior to the start of treatment in many low-to-middle income country settings, and inappropriate treatment with the 1st-line drugs can lead to treatment failure and additional resistance development. An entirely new regimen with a shorter treatment time and activity against both TB and MDR-TB would reduce this problem, providing appropriate therapy for the majority of TB cases. MDR-TB global spread focused attention on the critical need to develop new drugs, and new drug regimens, for TB, but especially for MDR-TB. SQ109 (in development by Sequella) and TMC207 (in development by Tibotec, a Johnson & Johnson company) are two clinical stage drug candidates that are now or will shortly be in Phase 2 efficacy studies in humans. Both have potent activity against MDR-TB clinical strains in vitro. TMC207 and Pyrazinamide (PZA), an established 1st-line TB drug, are synergistic in vivo and demonstrate excellent efficacy in a combination treatment regimen in a mouse model of TB. However, at least one additional drug will be needed to transition this regimen to humans, since PZA does not prevent the development of drug resistance to TMC207. The combination of SQ109 and TMC207 is also synergistic in vitro. We hypothesize that SQ109, with a completely novel mechanism of action, can be combined with TMC207+PZA to further improve treatment efficacy and reduce relapse. This proposal outlines an important research program between 2 companies with interesting drugs in clinical development, neither registered for use in humans. The results of these studies will give both companies a better understanding of combination drug efficacy and may identify a regimen that could be evaluated in the clinic. We propose to evaluate interaction of SQ109 and TMC207 in animal models of TB, with and without PZA, to generate nonclinical data on the interaction of these 3 drugs. Specific Aim 1. Evaluate drug-drug interactions of proposed combination treatments. Specific Aim 2. Evaluate efficacy of combination treatment regimens with these drugs. Specific Aim 3. Evaluate relapse rate of mice for the most effective regimen(s) identified in Aim 2.